1. Technical Field of the Invention
The present invention relates to novel catalysts for the purification/treatment of gases, especially of gaseous industrial effluents containing sulfur compounds, particularly to recover elemental sulfur therefrom, and, more especially, to novel catalysts for the Claus reaction and/or the hydrolysis of organic sulfur compounds.
2. Description of the Prior Art
In the conventional Claus process, the recovery of sulfur from gases containing hydrogen sulfide and, optionally, organic sulfur compounds comprises two stages.
In a first step, the hydrogen sulfide is burned in the presence of a controlled amount of air to convert a proportion of the hydrogen sulfide into sulfur dioxide according to the following reaction mechanism (1):
H2S+3/2O2xe2x86x92H2O+SO2xe2x80x83xe2x80x83(1)
and then, in a second step, the gaseous mixture obtained is passed through converters, in series, containing a catalyst bed wherein the actual Claus reaction (2):
xe2x80x832H2S+SO2xe2x86x923/xSx+2H2Oxe2x80x83xe2x80x83(2)
is carried out.
The overall balance of the reaction is therefore (3):
3H2S+3/2O2xe2x86x923/xSx+3H2Oxe2x80x83xe2x80x83(3)
Furthermore, other than hydrogen sulfide, the gases may contain organic sulfur compounds such as CS2 and COS, which are generally stable in the catalytic converters and which contribute to an increase of 20% to 50% in the emissions of SO2 and of sulfur compounds into the atmosphere after the fumes have been incinerated. These very objectionable compounds are either already present in the gas to be treated, or are formed during the first oxidation step which is conducted at elevated temperature.
These compounds can be removed via several types of reactions, in particular by hydrolysis according to the mechanisms:
CS2+2H2Oxe2x86x92CO2+2H2Sxe2x80x83xe2x80x83(4)
CS2+H2Oxe2x86x92COS+H2Sxe2x80x83xe2x80x83(5)
COS+H2Oxe2x86x92CO2+H2Sxe2x80x83xe2x80x83(6)
this being to limit the discharge of toxic effluents at the downstream end of a sulfur plant.
These reactions are also effected in a catalyst bed, advantageously using a catalyst based on titanium, cerium oxide or zirconium oxide, or alumina.
They are generally carried out simultaneously with the Claus reaction (2) in such converters.
For this purpose, the catalysts employed must exhibit the best possible activities, as well as the best resistance to the stresses experienced during use, as a function of time.
Alumina is commonly employed as a catalyst for these applications. One of the principal reasons for its deactivation is due to the concomitant formation of sulfates at its face surfaces, which sulfates reduce its catalytic performance, frequently drastically.
It has now unexpectedly been determined that the extent of the sodium content of alumina is a primary factor in the deactivation thereof, especially by sulfate formation.
Accordingly, a major object of the present invention is the provision of novel alumina catalysts comprising an amount of sodium values which is within a certain judiciously selected range.
Briefly, the present invention thus features a catalyst for the treatment/purification of gases containing objectionable amounts of sulfur compounds and/or for the hydrolysis of organic sulfur compounds, essentially consisting of a catalytically active alumina which comprises sodium values, the sodium content of the alumina ranging from 1,200 ppm to 2,700 ppm of Na2O by weight thereof.